The present invention relates to a multi-layer coating having excellent adhesion and sliding properties, and production method thereof, which can be applied onto a part surface of tools or similar items, such as a metal die, that are used, for example, in environments ranging from hot to cold.
The development of solid lubricants has mainly proceeded for intended use in environments wherein lubricating oil could not be easily employed, such as high temperature, extreme low temperature, ultra high pressure, a vacuum and the like. Among such solid lubricants, there are those which can be applied to parts having a complex shape by being deposited as a coating onto the object to be treated by a process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Substances such as molybdenum disulfide and diamond-like carbon (DLC) are already being put to practical use.
Of these, molybdenum disulfide is being applied to many sliding parts to improve seize resistance, due to its superior sliding properties and the fact that it can maintain a low friction coefficient even in an air atmosphere of up to several hundred ° C. In particular, by depositing molybdenum disulfide onto a hard layer such as a nitride or a carbonitride which have excellent wear resistance, a multi-layer coating having excellent sliding properties can be produced which has wear resistance and seize resistance at the same time. A method has also been proposed which improves the adhesion and weatherability of a coating by incorporating a metal element into the molybdenum disulfide (JP-A-2000-001768).
However, molybdenum disulfide has the drawback of being an extremely soft substance, which when formed into a coating does not always possess sufficient adhesion with the object to be treated. For this reason, if a molybdenum disulfide coating is applied as is onto a sliding part, there is the serious problem that the coating is susceptible at an initial stage of use to peeling and thereby lost without exhibiting sufficient performance. In addition, molybdenum disulfide also has the drawback that it is highly hygroscopic, which causes lubricating performance to deteriorate.
The above-mentioned JP-A-2000-001768 describes measures against such problems. Namely, because molybdenum disulfide hardens if incorporated with a metal element, adhesion with a part will certainly improve when formed into a coating. However, not only is this adhesive strength not strong enough to be applied to a part which is subject to vigorous sliding, if the ratio of the metal element is too high the excellent lubricating properties that an inherent characteristic of molybdenum disulfide deteriorates. Further, not only are the excellent lubricating properties of molybdenum disulfide affected by moisture in the atmosphere, but the molybdenum disulfide properties cannot be said to be adequate for utilization merely with the measures disclosed in JP-A-2000-001768.
It is an object of the present invention to provide a multi-layer coating having excellent adhesion and sliding properties, and production method thereof, by improving on conventional molybdenum disulfide lubricating layers.
As a result of detailed experimentation and investigation relating to molybdenum disulfide layer carried out by the present inventors, the present inventors realized that if, for example, a nitride, carbonitride or other such compound to serve as a hard layer is formed under the molybdenum disulfide layer, which is a sulfide, sufficient adhesion cannot be achieved because the material properties (constituent elements, interatomic bonds, crystal structure etc.) of these layers are substantially different. Further investigation based on this perspective showed that in order to increase the adhesion of a molybdenum disulfide layer, it is preferable to increase the ratio of the metal element in the layer. This investigation also showed that adhesion and sliding properties can both be sufficiently attained in the same layer by gradiently varying the ratio of the metal element in the layer.
Moreover, the present inventors showed that the factors influencing the sliding properties of a coating do not only relate to humidity, which had been pointed out in the past, and that oxygen impurities in the layer also have a large influence, thereby arriving at their proposal for an optimal production method for resolving the problems.